The present invention relates to a bandwidth management method and circuit for assigning time slots to signal sources sharing a common channel, to a communication apparatus and communication system employing this bandwidth management method and circuit, and to a dual-queue network unit used in the communication system.
An example of a communication system in which the present invention is useful is a passive double star (PDS) system employing the asynchronous transfer mode (ATM). The PDS topology provides an efficient way to use optical fibers to link multiple optical network units, located on user premises, to a subscriber line terminal in a telephone switching office. ATM provides for efficient transmission of mixed communication traffic, including digital voice, video, and data signals.
A problem in a fiber-optic ATM-PDS system concerns the assignment of time slots to different signal sources. To avoid collisions, the optical network units transmit to the subscriber line terminal only when directed to do so by the subscriber line terminal, in time slots assigned by a bandwidth management circuit in the subscriber line terminal. A conventional bandwidth management circuit assigns time slots to each optical network unit at substantially regular intervals, the length of the intervals being inversely proportional to the bandwidth allocated to the optical network unit.
This arrangement works well for signals such as video signals that have a substantially constant bit rate, but is far less satisfactory for signal traffic generated by sources such as personal computers, that tend to transmit data in variable bursts. If bandwidth is allocated according to the average bit rate of a burst signal source, then during the long periods while the signal source is not transmitting, the allocated bandwidth will be unused, but when a burst does occur, the bandwidth will be inadequate and much data will be have to be queued at the optical network unit. The queuing causes unwanted transmission delays, and the optical network unit needs a large buffer memory to store the queue. If bandwidth is assigned according to the peak bit rate of a burst signal source, the buffering requirements and delays can be reduced, but the signal source will have to be assigned a large fraction of the system's bandwidth resources, limiting the number of signal sources that can be served.